Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into more desirable gases. When the engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals like palladium and rhodium are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often called "three-way" catalysts.
It is desirable, however, to operate gasoline engines under "lean-burn" conditions where the A/F ratio is greater than 14.7, generally between 19 and 27, to realize a benefit in fuel economy. Such three-way catalysts are able to convert carbon monoxide and hydrocarbons but are not efficient in the reduction of NOx during lean-burn (excess oxygen) operation. Efforts have been made in developing lean-burn catalysts in recent years. One deficiency of some of the conventional lean-burn catalysts is that they are based on zeolite materials which are less than durable at the elevated temperatures necessary for their efficient catalytic operation in the exhaust gas system. Lean-burn catalysts act to reduce the NOx through the use of hydrocarbons and carbon monoxide over a catalyst, the hydrocarbons and carbon monoxide hence being in turn oxidized.
Recent efforts to solve the problem of NOx in lean-burn systems have focused on lean-NOx traps, i.e., materials which are able to absorb nitrogen oxides during lean-burn operation and is able to release them when the oxygen concentration in the exhaust gas is lowered. Hence, these traps are used with engine systems which operate primarily in a lean air/fuel ratio, but then when it is desired to purge the traps of NOx, the exhaust entering the trap is made richer, particularly rich of stoichiometric. Typical of catalyst materials used in conventional traps are an alkaline earth metal like barium combined with a precious metal catalyst like platinum. European Patent Application 0613714A2 published Sep. 7th, 1994 discloses that platinum or palladium in various combinations with at least two ingredient materials of the alkali metals, alkaline earth metals, transition metals, or rare-earth metal are capable of storing or absorbing nitrogen oxides under exhaust conditions of excess oxygen and desorbing the NOx during stoichiometric or fuel-rich atmospheres. When the NOx is purged, it is expected that the NOx is oxidized over the precious metal to nitrogen and oxygen.
The widely held mechanism for this absorption phenomena is that during the lean-burn operation the platinum first oxidizes NO to NO.sub.2 and the NO.sub.2 subsequently forms a nitrate complex with the other material, e.g., the barium. In the regeneration mode as during a stoichiometric or rich environment, the nitrate is thermodynamically unstable, and the stored NOx is released. NOx then catalytically reacts over the platinum with reducing species in the exhaust gas like HC and CO to form O.sub.2 and N.sub.2. Hence according to one strategy for using lean-NOx traps, a hybrid-mode engine strategy is used to cycle the air/fuel ratio between extended periods of lean operations where the traps sorb NOx emissions, alternated with brief, fuel-rich intervals to desorb the adsorbed NOx and regenerate the lean-NOx trap. U.S. Pat. No. 5,473,887 discloses such operation of an exhaust purification device, the teachings of which are hereby expressly incorporated by reference herein.
The alkali metal and alkaline earth metals which are typically utilized for NOx sorption have, however, the serious drawback that they are readily poisoned by sulfur in the exhaust gas. Most fuels for automotive vehicles contain sulfur and when burnt, the sulfur is converted to sulfur compounds like SO.sub.2. Over time, the sulfur compounds react with these alkali metal or alkaline earth trap materials forming sulfates which will not revert back to the sorption material. These sulfates are inactive for NOx sorption. The alkali metals are particularly problematic. As a result, the typical NOx trap material which uses precious metal and an alkaline earth like barium is strongly deactivated by sulfur in the fuel.
In concurrently filed patent application mentioned above, we found that a platinum/alumina catalyst, without inclusion along with the platinum of basic oxides like BaO or SrO reported to be an essential part of the catalyst, performs equally well under lean-rich cyclic conditions so that they can be advantageously used as NOx trap catalysts. Even though the Pt-BaO/alumina catalyst, when fresh, has a slight advantage over Pt/alumina, BaO-Pt/alumina is more sensitive to sulfur poisoning and has been found to lose this advantage upon aging. Since alumina is not a conventional basic oxide, the observation that a platinum/alumina catalyst free of an alkaline earth like barium oxide could be used as a NOx trap catalyst was totally unexpected to the inventors. At this point the inventors do not understand the mechanism of nitrogen oxide absorption by the Pt/alumina catalysts, however, it does not seem to be based on the basicity of the washcoat alone.
According to the present invention, we have now found that the incorporation of tungsten oxide into the Pt/alumina alkaline earth-free catalyst composition enhances its nitrogen oxide reduction at low temperatures under nitrogen oxide trap situations of cyclic lean-rich exhaust conditions as shown in FIG. 1. Since tungsten oxide increases the acidity of the catalyst, it would have decreased the basicity and thus would be expected to lower the nitrogen oxide conversion, if the nitrogen oxide absorption-desorption mechanism reported in the literature for NOx trap materials were completely correct. The inventors believe that the improved nitrogen oxide conversion found according to the present invention is contrary to the prevailing thought and therefore they found it quite unexpected. The formulation of tungsten and platinum on alumina was found by the inventors to be useful as a lean-burn catalyst as disclosed in related U.S. patent application Ser. No. 08/662,178 filed Jun. 12, 1997 mentioned above. Lean-burn catalysts are formulated to be used to continuously convert exhaust gas components during operation of the engine. They are not expected to store the components of the exhaust gas, like nitrogen oxides, during operation. Other well known lean-burn catalysts include zeolite materials. Such materials are not found useful as nitrogen oxide trap materials. Hence, it was unexpected that a tungsten oxide/platinum on alumina catalyst material would be suitable as a nitrogen oxide trap material.